A variety of materials, e.g. polymers and thickening agents, have been employed in the electroless deposition art to modify plating solutions. In certain cases the amount of additive has provided low viscosity plating solutions with enhanced properties. For instance, Shipley in U.S. Pat. No. 3,329,512 discloses low viscosity solutions for electroless deposition of copper which contain polymeric brighteners, e.g. cellulose ethers, hydroxyethyl starch, polyvinyl alcohol, polyvinylpyrrolidone, peptones, gelatin, polyamides and polyacrylamides which improve the quality of the deposit. Shipley also discloses the preparation of a 5% aqueous solution of low viscosity grade of hydroxyethyl cellulose having a viscosity of 75-150 cps; in Examples 1-4 Shipley adds the hydroxyethyl cellulose polymer solution at levels of 0.3 g/l (0.03%) providing plating baths with low viscosity, e.g. less than 10 cp, and low levels of copper, e.g. about 0.04 moles/liter. The bath of Example 43 contains a high level of copper, e.g. 0.27 moles/liter, and low levels of polymer, e.g. 50 ppm (0.05 g/1).
Morishita discloses in U.S. Pat. No. 4,099,974 low viscosity electroless copper solution containing low molecular weight (e.g. less than 6000) polyethylene glycol.
Goldstein discloses in U.S. Pat. No. 4,265,943 low viscosity electroless copper deposition solutions containing low levels, e.g. about 250 ppm (0.025%), polyethylene glycol or polyoxyethylene, which tend to slow the deposition rate.
Nakaso et al. disclose in U.S. Pat. No. 4,548,644 low viscosity electroless copper deposition solution containing 0.1 to 5 g/l of polyoxyethylene ether as a surfactant.
Sommer in U.S. Pat. No. 4,581,256 discloses low viscosity electroless plating baths containing 0.1-20 g/l of polysaccharides, e.g. sodium alginate, acacia, pectin, sodium alpha-glucoheptonate and gelatin at levels of 0.5 g/l.
Polymeric and inorganic thickeners have also been utilized in the electroless plating art for catalyst solutions, e.g. to provide catalytic inks that are amenable to silk screen printing applications. For instance, Heymann et al. discloses in U.S. Pat. No. 4,253,875 a catalytic lacquer for application by silk screen printing comprising an aqueous solution of a binding agent, a metal salt, a complex former such as EDTA, a reduction agent such as formaldehyde and, optionally organic solvents, stabilizers and fillers with thixotropic properties. Seeding with palladium is not required. The applied lacquer is dried by longtime drying at room temperature or by heating to 400.degree. C. to provide a seed layer of the metal salt which is strengthened by immersion in a conventional metal depositing bath.
For other examples of colloidal catalytic solutions see U.S. Pat. Nos. 4,048,354; 4,220,678; 4,224,178 and 4,273,804 where Feldstein discloses catalytic solutions for initiating electroless plating comprising colloidal metal, e.g. hydrous oxide colloids of copper or nickel, stabilized with a secondary colloid such as gelatin or gum arabic.
In many cases it is desirable to apply an electroless deposition solution to a surface which is not amenable to immersion in a plating bath, e.g. because the substrate is not stable in aqueous solutions, because the substrate is large or fixed in place in a way that prohibits immersion in a solution or because it is desirable to restrict the application of plating solution to the region of a catalytic image. In such cases it would be useful to employ a highly viscous electroless plating solution that would be substantially immobilized when applied to a substrate, i.e. would not run from the localized area of application. A common belief in the field of electroless plating solutions is that plating baths must be well agitated to allow sufficient mass transfer of metal to a catalytic surface and liberation of hydrogen from the plating surface. For instance, if hydrogen, which is liberated during the reduction of ionic metal to deposited metal, is not removed from the surface, the transfer of ionic species to the surface is impeded. Such a belief has no doubt inhibited the development of highly viscous plating media.